In the prior art, there are described many different ways for non-invasively measuring current flowing in a conductor. One well known method is to use what is known as a direct current transformer which typically uses a relatively low frequency excitation current and using a two-core method. In such a two-core method, a DC current through the two cores biases each with flux at the same polarity, while the flux in one core due to a modulator drive remains out of phase with that in the second. Each core reaches a saturation flux level so that it reaches saturation flux level at a different point in the excitation cycle for one alternation than for the other alternation. The result is an output signal at even harmonics of the excitation frequency.
Techniques such as the above are used to provide an absolute measurement of current. In many cases however, such as monitoring insulation breakdown in high current conductors, it is required to measure leakage currents in the presence of very high drive currents. For instance, DC motors on locomotives might have a drive current of 3000 amps and the leakage current desired to be monitored for insulation breakdown may well be in the order of milliamps. In such cases, the large drive currents result in very large magnetic fields and, even though send and return conductors may carry almost the same current, it is clear that the fringing magnetic fields which in this case will not be insignificant, would not cancel and will be dependent on the position of the conductors within the cores. There would therefore result a positionally dependent measurement and no coherent information would be recoverable. Some manufacturers have sought to obviate these problems by utilising complicated bus bar geometry so as to cancel out the fringing magnetic fields. However, apart from being costly such solutions are often not practical.